1. Field of the Invention
The present invention relates in general to a flow volume control device for delivering operating fluid from an engine-driven pump to a power assisted steering device through a throttle element and for returning an excess part of the fluid to an inlet area of the pump by way of the bypass passage. More particularly, it relates to a flow volume control device of the type mentioned above which is capable of decreasing the volume of fluid supplied to the steering device as the rotational speed of the pump increases.
2. Description of the Prior Art
Generally, a pump device used together with a power assisted steering device in a motor vehicle is provided with a flow volume control valve. The degree of opening of the valve is increased as an increase in the driving speed of the motor vehicle causes the rotational speed of a pump of the pump device to increase. This results in bypassing more of the pressurized fluid from an exhaust area to a suction area of the pump so that the flow volume of pressurized fluid to the steering device can be maintained approximately constant.
It is a general requirement to increase the steering reaction felt by the vehicle driver during high speed driving. To this end, an attempt has been made to decrease the flow volume of fluid to the steering device as the rotational speed of the pump is further increased beyond a predetermined value.
However, most known flow volume control devices of this type are designed to variably control the opening of a throttle passage in dependence upon the displacement of a valve spool. This makes it possible to obtain a predetermined characteristic of flow volume drop while no load is applied to the power assisted steering device (that is, while the same is not manipulated), but permits the valve spool to restrict the flow of the bypass fluid when the manipulation of the steering device causes a pressure elevation in the fluid system. The restriction of the bypass flow, however, causes the volume of fluid to the steering device to increase rather than decrease with an undesirable result that the strengthened steering reaction is weakened.